In the creators' commentary regarding his introduction, it was revealed that he was a design created by series illustrator, Shiroi Eiji, as a "dark Haohmaru". Unlike Rera, he is a separate entity from Haohmaru and shouldn't be confused as a part of Haohmaru's subconscious.
He is voiced by Tsuguo Nakajima and Shinsuke Fukui.
He is a bloodthirsty demon from the dark world of Makai, which was opened up by Shiro Tokisada Amakusa many years ago. Rasetsumaru can never assuage his thirst for killing. Because he doesn't clean his blade, it has turned the color of blood from the hundreds of people he has killed. He learns the existence of Haohmaru, an incredibly strong samurai who looks like him, and tries to find him in Hinowa to kill him. He finds Haohmaru at a temple a year later, but it is really a nun (implied to be Yumeji Kurokouchi) disguised as an illusion of him. Claiming him to be his/her final opponent, Yumeji decapitates him.
Rasetsumaru is a wild demon with an uncanny thirst of blood. He likes not only to kill, but also to make his victims suffer.
- Hurricane Attack - Rasetsumaru can fire a small hurricane, but with possibly more control compared to Haohmaru's style of this type of skill.
- Energy Projectile - Rasetsumaru can fire a small range projectile of energy.
- Energy Attacks - Rasetsumaru can fuel his slashes with energy.
- Berserker - By cutting himself, Rasetsumaru sacrifices a portion of his health in exchange for increased damage or more energy.
Parallel to Rera's fighting style with Nakoruru, Rasetsumaru is the former "Rasetsu/Bust" form of Haohmaru. He mirrors several of his counterpart's normal attacks but has a different move set, especially with a more limited arsenal that focuses on more erratic tactics such as extra sword-swings and more brute force.
The techniques he shares with Haohmaru are variations of his Senpuu Retsu Zan and Kogetsuzan. His Senpuu Retsu Zan: Setsu (Whirlwind Rending Slash: Temple) flies up from ground at a diagonal arc, and his Nagi Yaiba (Calm Blade) acts as a reversal. His control over the Senpuu Retsu Zan seems greater than the former as he possess a similar technique known as Senpuu Ha (Whirlwind Wave), where he slashes along the ground for a windy-dust blast. He also possesses a variation of the technique as his primary finisher, the Tenha Dankuu Retsu Zan (Heavenly Supreme Severing Air Rending Slash).
His Shitatari (Death Curse) can also fill up his power gauge at the cost of a portion of his own health, as Rasetsumaru even resorts to cutting himself.
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